In recent years, an increase in size of a wafer has been demanded, and a wire saw is mainly used for slicing an ingot with this increase in size.
The wire saw is a device that allows a wire (a high tensile steel wire) to travel at high speed, presses a workpiece (there is, e.g., a silicon ingot. It may be simply referred to as an ingot hereinafter) against the wire while apply slurry to the wire, and slices the workpiece to provide many wafers at the same time (see Patent Literature 1).
Here, FIG. 4 shows an outline of an example of a conventional general wire saw.
As shown in FIG. 4, a wire saw 101 is mainly constituted of a wire 102 configured to slice an ingot, a wire guide 103 around which the wire 102 is wound, a tension imparting mechanism 104 configured to impart tension to the wire 102, ingot feeding means 105 for feeding the ingot to be sliced, a nozzle 106 configured to supply slurry provided by dispersing and mixing abrasive grains such as SiC fine powder in a coolant, and others.
The wire 102 is reeled out from one wire reel 107, and enters the wire guide 103 through the tension imparting mechanism 104 formed of a powder clutch (a constant torque motor 109), a dancer roller (a dead weight) (not shown), an others through a traverser 108. The wire 102 is wound around this wire guide 103 for approximately 300 to 400 times, and then taken up by a wire reel 107′ through the other tension imparting mechanism 104′.
Further, the wire guide 103 is a roller provided by press-fitting a polyurethane resin around a steel cylinder and forming grooves on a surface thereof at a fixed pitch, and enables the wound wire 102 to be driven in a reciprocating direction by a drive motor 110 with a predetermined cycle.
Furthermore, the nozzle 106 is provided near the wire guide 103 and the wound wire 102, and the slurry can be supplied to the wire guide 103 and the wire 102 from this nozzle 106 at the time of slicing. Moreover, after the slicing, it is discharged as waste slurry.
Such a wire saw 101 is used, appropriate tension is applied to the wire 102 by using the tension imparting mechanism 104, the wire 102 is allowed to travel in the reciprocating direction by the drive motor 110, and the ingot is sliced while supplying the slurry, thus providing a desired sliced wafer.
At the time of repeatedly slicing a plurality of ingots by using the wire saw, according to a general ingot slicing method, slicing is carried out under conditions that a wire new line feed amount remains the same in slicing of all the ingots.
In general, a wire new line feed amount at a slicing start portion of the ingot and a wire new line feed amount at a slicing end portion of the same are values smaller than a wire new line feed amount at the time of slicing a central portion. It is to be noted that, in case of an ingot having a diameter of, e.g., 300 mm, the slicing start portion of the ingot is a portion that is 15 mm from a portion where the wire first touches an outer peripheral end of the ingot, and the ingot central portion corresponds to a portion that is 150 mm from the outer peripheral end.
Moreover, when a thickness distribution of a wafer sliced out from the ingot is confirmed, a thickness of the wafer at the slicing start portion of the ingot is smaller than that of the central portion. As described above, in slicing of the ingot using the wire saw, the slicing start portion has the smallest thickness. A difference between a thickness of the slicing start portion and a thickness of the central portion within a wafer surface will be referred to as thickness unevenness hereinafter.
A phenomenon that the thickness of the slicing start portion is reduced is caused due to a large wire diameter of the wire used in the slicing start portion. To decrease the wire diameter of the wire, abrading the wire can suffice. As a method for adjusting an abrasion amount of the wire, there is a method for changing a wire new line feed amount. The abrasion of the wire is reduced when the wire new line feed amount is increased, and the abrasion of the wire is increased when the wire new line feed amount is reduced.
Thus, as a method for eliminating a problem that the thickness of the slicing start portion is reduced, there is a method for reducing the wire new line feed amount at the time of slicing the slicing start portion, and hence the thickness unevenness can be decreased. Additionally, when the wire is wound back before slicing the ingot and an abraded portion that has been already used is used for slicing, the thickness unevenness can be reduced.